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Shrinking fish more vulnerable to predators

SALLY SARA: A new study by the CSIRO (Commonwealth Scientific and Industrial Research Organisation) shows that fish around the world are shrinking in size.

Commercial fishing and climate change are believed to be the main culprits behind the decrease, but CSIRO scientists say the shrinking size is also leading to a drop in fish populations, with smaller fish more susceptible to predators.

Jennifer Macey reports.

JENNIFER MACEY: Scientists at the CSIRO have found that the shrinking size of fish may be making them more vulnerable to predators.

Dr Asta Audzijonyte, from the CSIRO's Marine and Atmospheric Research unit in Tasmania, says fish are getting smaller due to climate change and fishing. But she found that even small decreases in the body size of some fish species had a big impact on their population size.

ASTA AUDZIJONYTE: So basically, if the flathead gets a bit smaller, it's being eaten more heavily by other fishes like *barracouta, which is the predator of the flathead.

The big flathead actually can eat small barracouta, so normally, flathead would eat small barracouta, and large barracouta would eat small flathead.

However, when flathead get smaller, it cannot eat as many barracoutas, so there are more and more larger barracoutas that eat more and more flathead, and in this way we get such a loop, which makes flathead even smaller and even less abundant than before.

JENNIFER MACEY: While there has been a lot of evidence from European and North American fisheries about shrinking fish sizes, the CSIRO looked at five Australian fish species. Tiger flatheads, jackass morwong, silver warehou, blue grenadier and pink ling.

Dr Audzijonyte says fishing could be changing the long-term evolution of fish sizes.

ASTA AUDZIJONYTE: We usually selectively catch the largest and the fastest-growing individuals, leaving the small ones in the ocean. So we only have the small ones left.

But then also, only the small ones get a chance to reproduce, and they are more likely to produce other small and slow-growing fish.

So in this way, we are getting a long-term evolutionary decline in body size, which is more problematic because the fish will not get bigger quickly, even if we completely stop fishing, because they already became evolutionarily smaller.

JENNIFER MACEY: The scientists used a complex modelling system to test how the decreasing fish size affects the interactions between different species.

Professor Jessica Meeuwig from the University of Western Australia's Oceans Institute says this is the strength of this particular study.

JESSICA MEEUWIG: So for instance, being smaller might mean that you have more difficulty moving long distances, travelling between locations, but if on top of it, it means that you're more vulnerable to getting eaten, that's a double whammy. And some of those feedback loops are unexpected.

JENNIFER MACEY: She says recent moves to establish a network of marine reserves around Australia's coastline may help species recover.

JESSICA MEEUWIG: I think the message here is that we need to be extremely conservative about the amount of fishing quota that's allocated.

The other solution I think there too is if we actually really want to test some of these ideas in the field, having large marine sanctuaries where there is no fishing allows us to actually compare, for instance, the size distribution of fish populations and not fish populations, and how that relates to some of these feedback loops.

JENNIFER MACEY: The study is published in the latest edition of the science journal Biology Letters.

SALLY SARA: That's Jennifer Macey with that report.

*EDITOR'S NOTE (March 1, 2013): The original transcript incorrectly named the example predator fish as "barracuda" when it should have been "barracouta". The transcript has been corrected.